The safety of calorie restriction in pregnancy is not known and genuine concerns exist around infant psychological or physical development.
Long- term follow-up of children born to mothers exposed to famine suggests that future health is compromised.
Infants born to previously well-nourished Dutch women restricted to 800 kcal/day in late pregnancy during the five months of famine in 1944/5 developed normally, although thinner at birth and at 18 years (64).
However, when middle-aged these children had a higher incidence of glucose intolerance and diabetes (65).
Maternal ketosis, induced by calorie restriction, has been implicated to impaired foetal neuro-physiological and cognitive development (66,67).
While there is a general reluctance to recommend severe calorie restriction in pregnancy even in obese women, modest calorie constraint for those with GDM may be safe as these women are relatively protected against ketosis by their high hepatic glucose outputs (54,68,69).
Theoretically maternal ketosis can be lessened during modest calorie restriction when small frequent meals containing slowly absorbed carbohydrates are taken, as such diets are associated with an attenuated insulin response that delays lipolysis and ketogenesis (70).
We have previously reported that when the daily energy is restricted to 20, 25 kcal/kg/day for obese women with GDM (pre-pregnancy BMI>28 kg/m2) from the 24th week of gestation, weight gain is half that of women with a similar pre-pregnancy weight who receive no dietary intervention, and their risk of delivering an LGA infant is similarly reduced (71).
This degree of modest calorie restraint has also been shown to improve glycaemic control (69).
Frequent small meals containing slowly absorbed carbohydrates help to prevent ketosis.
All women receiving a diet that is calorie restricted should have regular foetal ultrasound examinations to ensure that foetal growth is not compromised.
THE OPTIMAL MIX OF DIETARY CARBOHYDRATE AND FAT FOR GDM
The diet for the diabetic mother needs to limit excess maternal, foetal transfer of glucose. As post-prandial hyperglycaemia is the time of maximal maternal, foetal glucose transfer, treatment interventions need to target this period (6). Controversy exists on how best to achieve this. Some authorities recommend limiting carbohydrate at the expense of increasing dietary fat, while others
THE DIETARY MANAGEMENT OF DIABETIC PREGNANCIES 101
favour high-carbohydrate diets with a low glycaemic response.
It is the authors’ belief that promoting diets that actively limit carbohydrate over fat sends out the wrong lifetime educational message.
Clinical studies suggest that it is the type of carbohydrate and fat rather than the absolute amount that dictates the glycaemic and metabolic responses to a meal.
As a degree of gastric stasis is common in pregnancy, the glycaemic response of many carbohydrates is blunted.
The American Diabetic Association (62) recommend limiting carbohydrate to 40% of the total energy content by increasing dietary fat to 40%.
This advice is based on clinical studies showing women with GDM have better glycaemic control when consuming less than 45%, rather than more than 45%, of their calorie intake as carbohydrate (72,73).
The American approach gives no acknowledgement to the fact that different ingested carbohydrates have different glycaemic responses as measured by their glycaemic index (74).
British advice on the diabetic diet in pregnancy does not recommend limiting carbohydrate to 40% of the total energy and indeed suggests this figure should be nearer 55%, with the majority of carbohydrate having a low glycaemic index (75).
Low glycaemic index diets can in fact increase insulin sensitivity in both pregnant and non-pregnant individuals (42, 44,76).
In pregnancy glycaemic control deteriorates when refined carbohydrate contributes more than 45% of the total energy (72).
By contrast when refined carbohydrates are exchanged for low glycaemic index carbohydrates, 60% of the total dietary energy can be consumed in this form without any change in glucose tolerance (42, 44).
As the glycaemic response to rapidly absorbed refined sugars is greatest in the early morning, advice on suitable commercial breakfast cereals should be given (77).
DIETARY FAT
The short-term dietary studies that demonstrated a benefit of high-fat versus high-carbohydrate diets on post-prandial blood glucose values (72,77) may, as discussed above, have been accounted for by the use of high glycaemic index carbohydrates in these studies.
Jovanovic’s group (78) have also stated that the addition of dietary saturated fat to a test meal produces a significantly lower glycaemic and insulin response than when the test meal contains the equivalent proportion of monounsaturated fat.
The differences may be explained by slower gastric emptying when the meal contains a high saturated fat content.
However, we believe that even if the glycaemic response mid-morning can be lowered by increasing the saturated fat content of the breakfast, advocating such a diet to women at future risk of diabetes and cardiovascular disease remains highly questionable, when epidemiological and clinical studies show that high-fat diets are associated with insulin resistance, b-cell dysfunction, and
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recurrent GDM pregnancy and future diabetes (56,59).
Also the long-term effects of a high maternal saturated fat diet on cardiovascular health is unknown.
Animal studies certainly suggest caution as high-fat diets in pregnant rodents can promote cardiovascular disease in the next generation (79,80).
Increasing the saturation content of the diet in pregnant rats leads both to unfavourable changes in fatty acid compositions and function of the major arterial vessels.
High-fat diets in pregnancy have also been associated with severe hyperemesis gravidarum, with a 5.4-fold increased risk reported for every additional 15 g/day of dietary saturated fat (81).
Increasing the polyunsaturated fat (PUFA) content of the diet while restricting the saturated fat may provide an alternative approach to safely reducing the overall dietary carbohydrate content.
A large epidemiological study in China reported that a high habitual intake of dietary PUFA with a correspondingly raised low dietary polyunsaturated to saturated fat ratio protected against gestational diabetes (57).
It remains to be proven whether Western women would achieve a similar benefit, as their PUFA intake is highly correlated with saturated fat intake.
The potential benefits of increasing monounsaturated fat (MUFA) intake in pregnancy still need to be shown.
A recent small Danish study failed to show any improvement in insulin sensitivity in late pregnancy when women with GDM eat diets high in MUFA rather than high in carbohydrates, although a favourable effect on blood pressure was reported (82).
Outside pregnancy improved insulin resistance and lipid profiles have been reported when either a high-carbohydrate diet or a monosaturated-enriched diet replaces dietary saturated fat, with reductions in plasma LDL cholesterol observed (83,84).
If high-MUFA diets are to be promoted over a high-carbohydrate diet, one needs to ensure that overall calorie intake leading to unnecessary weight gain does not occur (85).
A large Swedish epidemiological study has suggested that increasing dietary long-chain n-3 fatty acids (omega-3 fatty acids) by increasing fish and fish oils may provide some protection against low birth weights and pre-term deliveries (86).
Similar diets in Type 2 diabetic subjects have been shown to have some favourable metabolic effects on serum triglycerides but not plasma LDL cholesterol (87,88).
Other food sources of n-3 polyunsaturated fatty acids include flaxseed and flaxseed oil, canola oil, soybean oil and nuts.
Population studies suggest that foods containing n-3 fatty acids, specifically eicosapentaenoic acid and docosahexaenoic acid, may provide long-term cardio-protection (89,90).
There are therefore potential theoretical benefits for increasing dietary long-chain n-3 fatty acids in diabetic women both in and out of pregnancy.
In the face of no real clinical-based studies on the optimal ratio between saturated, poly, mono and fish oils for pregnancy, it is our policy to aim for a ratio of sat:poly:mono of 1:1:1, with the specific advice to eat oily fish three
THE DIETARY MANAGEMENT OF DIABETIC PREGNANCIES 103
times a week (91). These recommendations are similar to those for people with diabetes and coronary heart disease.
DIET AND INSULIN THERAPY FOR GDM
Once diet alone can no longer consistently ensure fasting glucose values below 5.5 mmol/l and a 1 h post-prandial value below 7 mmol/l, the introduction of insulin should be considered (63).
It is important to recognise that a small proportion of women will require insulin early in pregnancy and not to assume dietary non-compliance (92).
Those requiring insulin are the most metaboli- cally compromised and tend to have both the highest perinatal complications and the fastest deterioration to diabetes after pregnancy (93).
Insulin is also occasionally introduced in later pregnancy for obstetric rather than glycaemic reasons; this might occur for accelerated foetal growth or unexplained polyhydramnios (94).
It is important to stress that once insulin is introduced for the management of GDM the dietary management remains equally important.
The need to limit weight gain remains for obese women who now need to balance this with having sufficient carbohydrate snacks throughout the day to prevent hypoglycaemia.
Although short periods of hypoglycaemia are not detrimental to the foetus they are unpleasant for the woman and frequently result in sudden rises of blood sugar due to the action of counter-regulatory hormones and the consumption of sugary drinks.
Frequent episodes of hypoglycaemia often result in women chasing these high-rebound glucose levels by increasing their insulin dosage, which can result in further hypoglycaemic attacks and unnecessary weight gain.
When starting on insulin women should be advised to take low glycaemic index carbohydrates at meal times and for snacks between meals and before bed.
Fruit is ideal for snacks as it is low in fat and calories.
Fruit, by being slowly absorbed, reduces the risk of hypoglycaemia while allowing post- prandial glucose levels to be lowered without having to increase the insulin dose.
LONG-TERM DIETARY ADVICE FOR THE MOTHER AND HER CHILD
As most women with GDM are obese and all have at least one child at increased risk of adolescent obesity and diabetes, providing dietary education and advice that extends beyond the pregnancy is extremely important. Lifestyle changes encompassing diet and exercise have been shown to reduce the risk of GDM in subsequent pregnancies as well as delaying the progression to Type 2
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diabetes (59,95,96).
Women with a history of GDM are an ideal group to target, not only because of their own heightened risk of future diabetes (97,98) but to ensure a healthy lifestyle within the family unit, hence reducing the risk of obesity and future diabetes in the children also.
Ideally all women with GDM should receive lifestyle advice and education in pregnancy that is relevant to after pregnancy.
It will be an important challenge to find methods of delivering dietetic education and advice both effectively and cheaply to enable all women with GDM to receive the necessary ongoing support and care they require after pregnancy in the community.